Background

An integrated program in Mining Engineering is offered through the Robert M. Buchan Department of Mining and the Faculty of Engineering and Applied Science at Queens University in Kingston, Ontario, Canada. There are three study areas within the program - Mining, Mine-Mechanical and Mineral Processing. As part of the fourth year curriculm, a group of forty five students and educators travelled to Chile to visit a variety of mining operations in February, 2010. Punta del Cobre was one operation that was visited during the extensive tour and a synthesis of its technical design and operation follows.

Geography

Location

The Punta del Cobre Mine is located in the Atacama Region of Chile. The underground operation is approximately 3 km from Tierra Amarilla, a community located in the Copiapó Province. The mine is currently being operated by Pucobre, or Sociedad Punta del Cobre S.A., a Chilean based mining company. Pucobre focuses on operations in the Atacama region, in northern Chile.

Climate

The majority of the company’s major mining activities are in the Copiapó valley, which is in the heart of the Atacama Desert. The average elevation of the region is approximately 600 meters above sea Level. The Atacama Desert has been referred to as the driest place on earth and last year, the mine received only 17mm of rain in three minutes during a single rainfall. [1] The desert stretches from a thin coastal shelf along the Pacific Ocean, up to the foothills of the Andes and much of the region consists of virtually lifeless plains. Photos of the surface conditions at Punta del Cobre are shown in Figures 1 and 2. [1]

There are however highly productive river gorges which are layered with rich mineral sediments from the Andes such as the Elqui Valley close to La Serena, Chile. These areas produce bountiful harvests of avocados, lemons, tomatoes and a variety of other fruits and vegetables. Chile is very well known for its wines and the national drink (Pisco), is also produced from grapes grown in these areas. A photo of the Elqui Valley, is shown belown in Figure 3. [3]

There are over a million people living in the Atacama today, primarily in coastal cities, mining establishments, fishing villages, or small oasis towns. [4]

Geology

Local Area

The Punta del Cobre district is an iron oxide-rich Cu-Au-Zn-Ag deposit approximately 5km wide and 20 km long located near Copiapo, Chile. The ore occurs in veins, breccias and stringer bodies and is hosted by volcanic and volcaniclastic rocks. There is one central fault line that runs through the Pucobre mine that separates the orebody by 10m. [1]A map of the geology in the Candelaria-Punta del Cobre, Chile district is shown in Figure 4 and a cross section is shown in Figure 5.

Minerals

The ore at Punta del Cobre is a hypogene assemblage of pyrite, magnetite or hematite and chalcopyrite. The minerals associated with ore in the shallower areas of the Punta del Cobre district are biotite- potassium feldspar oralbite-chlorite ± calcite alteration while in the deeper areas the ore is hosted in biotite-potassium feldspar ± calcic amphibole ± epidote alteration.[6]

Operations

The company makes an effort to make sure that they are utilizing the very latest technology in fields of, transportation, loading, electro-hydraulic tunnel boring equipment, blasting, and production methods. Pucobre has been making a constant effort to improve the safety and efficiency of their operation. Current efforts are being placed on the acquisition of modern loaders, trucks, and drilling equipment. Remote control LHDs, and higher capacity LHDs have been purchased over the past few years in an effort to upgrade and modernize the fleet beyond the latest industry standards. The control unit for a remote LHD is pictured in Figure 6. Recently, significant investments have been made for the construction of new, and the fortification of old refuge shelters. The site employs 510 people including contractors and the professional staff includes areas of geology, maintenance and mining operations. [2]Figure 6 - Remote Control LHD Operator[2]

Mining Method

Mine Locations

Annually, the entire operation produces concentrates of copper, ore and gold, and copper cathodes from three operating mines strategically positioned in the orebody to enable blending for mill processing. These three mines are known as Punta del Cobre, Mantos de Cobre and Granate. A map of these areas is shown in Figure 7.

Annual Production Rates

The Punta del Cobre underground operation currently contributes the most and generates 10 Ktpd while Mantos de Cobre contributes 4.5 Ktpd. [1] Granate, will also contribute a further 4.5 Ktpd when it is fully operational by 2013 (Punta del Cobre, 2010). This development will supplement decreasing tonnages in Punta del Cobre as the pit develops. Ore arriving to the mill from Punta del Cobre will drop from 48 million pounds in 2010 to 32 million pounds by the beginning of 2013. [1]The impact of this drop will be mitigated by Granate although a significant amount of oxide stripping needs to occur. Oxide stripping is projected to last until 2013 with annual targets of 25 million pounds. [1]The ore is processed in two plants, San Jose and Biocobre to produce both concentrates and cathodes at the same time. San Jose produces 120,000t of concentrates per year from 10Ktpd of ore, while Biocobre produces 9,000t of copper cathodes from 3,300 Ktpd of ore.[1] A summary table of projected production is shown in Table 1.

Mining Method

The geometric characteristics, regular boundaries of the ore body and stable structures and roofs have permitted the application of sublevel stoping. The Punta Del Cobre Mine is well known for having large open unsupported stopes due to the remarkably competent ground conditions that are unique to this mine. Essentially ore is extracted from each sublevel leaving open stopes without artificial fill or support. The sublevels are spaced at 30m intervals and are constructed in sequence to accomplish drilling, extraction and transportation of the ore.

Drilling is completed from open stopes, and after blasting it falls into a lower level. At this level the ore is removed using front end loaders and remote control LHDs. These in turn load articulated trucks to transport the ore to the surface for shipment to beneficiation plants. Loading and transport is accomplished directly at the production face without any intermediary steps.

Production Drilling

Production drilling at Punta del Cobre is of two types: radial upward at a diameter of 3 " (Top Hammer), and benching at 4.5" diameter (DTH). The mine design indicates that to exploit the orebody, 55% of resources will require radial drilling while the remaining 45% will be accomplished through down the hole drilling 4.5” in diameter. [1]

Radial upward drilling is completed with 2 meters of burden and 3.2m spacing at an angle of either 75⁰ or 90⁰. The 3” diameter is the largest size that can be effectively used to ensure good confinement of the explosive at reasonable cost. The length of the shots is determined by the area in the current design and maximum length of the shafts with a 3” diameter is 30m. This design enables Punta del Cobre to get yields of 10.5 tons per meter drilled, while drilling capacity is 5,000m per month.[1]

Benching down the hole drilling with a diameter of 4.5” allows a greater amount of explosive used per meter drilled. This in turn increases the size of the drilling grid from 3.5m to 4.5m for the spacing and burden. This increased mesh size brings additional benefits by reducing the number of shots drilled, increasing the rate of drilling to 24t/m drilling, a decrease in explosive load time since it is accomplished by gravity and smaller ore fragments after blasting. [1]

Explosive Use

Essentially, Punta del Cobre utilizes ANFO with a humidity of 2% for radial upward holes. The increase in moisture allows the explosive to adhere better to the walls of the borehole, allowing it to remain in place rather than falling down.

The explosive base is initiated by a 225g booster deep in the hole and an additional booster of 150g in the collar. The booster is in turn initiated by non-electric millisecond detonators varying between 4m and 27m long. .[1]These are in turn initiated by detonating cord at 3g / m which begins with a No8 current fuse. .[1] A similar technique for DTH is employed is also employed.

Principle Stresses

At the 365 level, the major principle stress exhibits strength properties of 30.05Mpa along an azimuth of 134.70⁰ and an inclination of 2.70⁰. [1] The intermediate stresses exhibit readings of 10.70 Mpa, along an azimuth of 32.10⁰ and an inclination of 77.90⁰.[1]Finally, the minor stresses are measured to be 6.12Mpa, with an Azimuth of 225.20⁰ and an inclination of 11.80⁰.[1]

At the 560 level, the major principle stress exhibits strength properties of 20.68Mpa along an azimuth of 310.00⁰ and an inclination of 6.30⁰.[1] The intermediate stresses exhibit readings of 13.65 Mpa, along an azimuth of 211.60⁰ and an inclination of 52.80⁰.[1] Finally, the minor stresses are measured to be 11.71 Mpa, with an Azimuth of 44.60⁰ and an inclination of 36.80⁰.[1]

The mine is well known for large open unsupported stopes that seem to defy generally accepted values for safe design as shown below in Figure 8.

Ventilation

The mine requires 800,000cfm of air to operate properly with a pressure of 10 pounds of water. [27] The system operates with two axial fans complete with electric motors at 1000Hp and 1 ventilation shaft 380m long and 6m in diameter.[1]The ventilation system is installed with a series of galleries, covers and doors to regulate the pressure and was designed to handle 417,000cfm per fan at 10.4 pounds of water.[1]

Communications

The radial communications system installed at Punta del Cobre is 10km in length and will eventually expand to cover 16km.[1] It currently features one main line, installed along the main access ramp and unfortunately is exposed to the impact of operating equipment such as trucks. The expanded system will feature three trunks, installed for redundancy to eliminate exposure to equipment and improve the system.

Access & Roads

Currently the mine is accessed by two main entry points 2km apart from each other. [1]The mine aims to develop 5m in horizontal distance by 4.15m in height, to create a ramp slope of 10%.[1] However, due to the dimensions of the access points, the characteristics of transportation equipment and traffic, access to the mine is currently limited to only one main entry point. It is therefore essential for safety reasons that if two vehicles are passing each other they must do so in an area not less than 150m in width. The roads are natural rock and are maintained with systematic treatments to reduce dust and maintain the roadbed.

Principal Mining Equipment

Punta del Cobre operates with a fleet of equipment from major manufacturers including Volvo, Sandvik, Atlas Copco and Bell. They are individually licensed to perform repairs on Volvo equipment and diagnose and repair all electrical components such as controllers and computers on site. The machinery shop is equipped with excellent facilities for diagnosing electronic equipment failures to perform these repairs and is stocked with supply of parts that are frequently needed in repairs. The principle equipment fleet is listed below in Table 1: Principle equipment fleet at Punta del Cobre.